The present invention relates to a coated paperboard for food, comprising one or more layers of fibre material and a heat-resistant polymeric coating, which gets into contact with food. Further, the invention relates to the manufacturing process and use of such a paperboard, and to a number of products obtained therefrom.
A heat-resistant oven board is used as material for such food dishes and packages, which have to withstand heating in a common or microwave oven. A typical heatable food dish is an oven dish, which may be used as part of a finished package for heatable food, but which may also be marketed to consumers as a separate product. The polymeric coating of an oven board makes the paperboard water and grease resistant, and the polymer has to withstand the temperatures used in heating, typically at least 230xc2x0 C.
It has been known to use polypropylene or polyesters, such as polyethylene terephthalate (PET) as coating for oven boards. The problem with some of these known polymers has been insufficient heat resistance. Another problem relating to all polyesters used as coatings is the poor adhesion of the polymer to the paperboard. In order to achieve sufficient adhesion, the coating has to be spread onto the paperboard in layer thicknesses which are at least approx. 35-40 g/m2. If the layer is thinner, there is a danger of it coming off, for example, due to vapour released from the paperboard. The use of possible binding agents improving the adhesion has been restricted by their poor heat resistance. Pin holes are also easily left in a thin polymeric layer.
Polyesters as such, and especially PET with a high melting point, are very well adapted to be used as coatings for oven paperboard. These polymers are compatible with food, because of their sufficient mechanical strength and heat resistance, and because their organoleptic advantageousness is kept at oven temperatures, they are heat-sealable and they may be spread onto the paperboard by extrusion. The only problem is caused by layer thicknesses which indicate a high material consumption and high amounts of polymer contained in the waste taken to dumping areas or material recycling. The need for reduction in polymer consumption is especially emphasised as even stronger and thinner fibre bases for paperboard have been developed and as the norms concerning waste take into account also the relative amount of polymer compared with fibre material included, besides the absolute amount.
The object of the present invention is to form a coated paperboard, in which the adhesion of the polymeric coating to the paperboard has been improved, while simultaneously making the coating thinner and less prone to hole formation as before. It is characteristic of the paperboard of the invention that it is formed of superimposed polymeric layers, comprising an outer layer in which the melting point for the polymer used is at least 230xc2x0 C., and an inner layer placed against the fibre material layer, the inner layer including polymer with a melting point of at least 230xc2x0 C. blended with another polymer achieving adhesion between the coating and fibre material, the melting point of this polymer being 130-185xc2x0 C.
The outer layer of the two-layer polymeric coating of the paperboard of the invention contacting the food thus comprises a heat-resistant polymer with the necessary oven properties and preferably consisting of some known coating polymer for oven boards that has been tested in use. According to the invention, adhesion of the outer polymeric layer to the paperboard, i.e. to the fibre material layer below, is achieved by an inner adhesion layer in which another adhesive polymer with a lower melting point has been blended with the heat-resistant polymer. Because of the heat-resistant polymer melting at or above 230xc2x0 C, the inner layer withstands heating in a stove or microwave oven at the same time as it is modified by said polymer melting at lower temperatures so that its adhesion to the fibre material is substantially better than before. Due to improved adhesion, the two-layer polymeric coating for the paperboard of the invention may be produced with a considerably smaller amount of polymer, even when added together, than the previous one-layer coatings for oven paperboards.
Besides oven boards, the paperboard of the invention is also applicable to liquid packaging boards, e.g. drinking cups and other disposable dishes, due to its compatibility with food and its heat-sealability.
The mixture ratio of polymers in the inner coating layer of the paperboard of the invention preferably is within the range of 85-97% of polymer with a higher melting point (xe2x89xa7230xc2x0 C.) and 3-15% of adhesive polymer with a lower melting point (130-185xc2x0 C.). The polymer with the higher melting point is most preferably the same polymer as has been used in the outer layer of the coating.
A preferable polymer to be used in the outer coating layer and also as the second polymeric component of the inner coating layer is polyethylene terephthalate, such as Eastapak PET copolyester with a melting point of 240xc2x0 C., manufactured by Eastman Chemical Company, or Eastapak PET polyester 12440, manufactured by the same company. In PET-based copolyesters, comonomers used in relatively small amounts (approx. 1-10%) may comprise, for example, cyclohexane dimethanol (CHDM) or isophthalic acid (IPA), which improve the processability of the polymer. Other polymers may comprise polybutene terephthalate, polyethylene glycol-modified polyethylene terephthalate (PETG), polyethylene naphthalene (PEN), and polypenthene, which, used as such, all adhere poorly to fibre material; but the problem of adhesion related with these may be solved with a two-layer coating of the present invention.
The adhesive polymeric component melting at 130-185xc2x0 C. for the inner coating layer may preferably comprise a terephthalate-based copolyester which especially well both blends with and binds to said polyethylene terephthalate melting at or above 230xc2x0 C. An example of such polymers is Eastobond copolyester 19411 manufactured by Eastman Chemical Company, in which the used comonomer is diethylene glycol (DEG), reducing the crystallinity of the polymer and increasing the polarity due to its OH groups, these factors improving the adhesion between the polymer and the paperboard.
The invention is firer applicable in an advantageous way so that, besides the two polymeric components, the inner coating layer is provided with fine mineral substance. It has been noted that mineral substance facilitates the coextrusion of polymeric layers onto paperboard and makes it possible for the polymeric layer to be even thinner than before. An especially suitable mineral is, for example, calcium carbonate but, for example, also titanium oxide or talcum are usable. When using a mineral component, the composition of the inner coating layer may preferably vary within the range of 80-90% of polymeric component melting at a higher temperature (xe2x89xa7230xc2x0 C.), 3-10% of adhesive polymeric component melting at a lower temperature (130-185xc2x0 C.), and 5-15% of mineral substance.
The total weight of a two-layer polymeric coating of the paperboard of the invention is preferably at most 25 g/m2, most preferably 15-22 g/m2. By using said mineral substance as part of the inner layer, the total weight of the coating may be in an even lower range, i.e. 13-22 g/m2.
The fibre base for the paperboard of the invention preferably consists of a three-layer paperboard, in which the middlemost layer is a thicker layer of a mixture of chemical pulp and CTMP, and the layers on both sides of this layer substantially consist of pure chemical pulp. The polymeric coating may be spread either on only one side or both sides of the paperboard.
It is characteristic of the method for manufacturing a coated paperboard of the invention that the polymer forming the outer layer of the coating and the polymeric mixture forming the inner layer are together coextruded onto a moving paperboard web. Extrusion is followed by rapid cooling during which the polymer remains in an amorphic state so that it is heat-sealable. Most preferable for coextrusion is a layer combination in which the outer layer to be placed onto the paperboard consists of polyethylene terephthalate, such as said PET polyester 12440 or PET copolyester 9921, the inner layer comprising a mixture of polyethylene terephthalate, such as either of the said products, of a terephthalate-based copolyester, such as the said Eastman copolyester 19411, and of calcium carbonate. However, in the applications of the process, the layers to be coextruded may vary, as is apparent of the above description of the paperboard of the invention.
Products to be manufactured of the paperboard of the invention comprise, above all, paperboard oven dishes for heating food in a common or microwave oven. For protecting the paperboard base of the dish and for preventing the food from sticking to the dish, the polymeric coating of the paperboard has to be provided at least to the interior surface of the dish. However, it is preferable to provide both the interior and exterior surface of the dish with a polymeric coating, in which case the paperboard is also protected from possible spatters and grease in the baking plate.
The oven dish of the invention may be formed of paperboard coated with polymer either by pressing or, alternatively, by folding and joint sealing the folds thus produced to the exterior surface of the dish. In either case, the polymeric coating withstands the joint sealing and other moulding measures without hole formation or breaking.
According to the invention, the oven dish may form part of a finished package for food, the package and its contents being heatable as such in a stove or microwave oven. Besides the dish and the heatable food packed into it, such a package comprises a separate protective cover or wrapping, which is removed before heating, when necessary.
As mentioned above, the paperboard of the invention is also usable in products manufactured of liquid packaging board, such as disposable cups, plates and packages. Here the advantage of polyesters, such as PET, is that they are heat-sealable and that they do not give any taste or smell to foods in contact with them. With the invention, it has been possible to improve the mutual adhesion of PET and paperboard and to reduce the thickness of the PET layer.
The invention further comprises the use of the coated paperboard described above as oven board or as liquid packaging board and, especially, the use of it as part of a consumer package shaped as a dish, containing heatable food.